The invention relates to a compressed-air system having a safety function, with a first and a second working valve, each of which has a working connection and a venting connection connected or connectable to the atmosphere, wherein the two working connections are connected in parallel fluid communication with one and the same load output of the compressed-air system which load output is connected or connectable to a load, wherein each working valve is switchable into a venting position connecting its working connection with its venting connection and wherein the two working valves can simultaneously assume their venting position. The invention also relates to a method for operating such a compressed-air system.
A compressed-air system having a design in the above sense is known from EP 1 645 755 B1 and contains as a particular aspect a safety function, ensuring what is known as two-channel venting of a load connected to the load output. This means that a load, for example an automation system, has two redundant working valves available for venting, so that if one of the working valves fails reliable venting can nevertheless be ensured by means of the other working valve. In the case of EP 1 645 755 B1 the safety function is integrated into a soft start device of the compressed-air system, with the help of which the compressed air supply to a load can be optionally switched on or off, wherein upon switching on a smooth pressure build-up is ensured and wherein switching off is accompanied by venting on the load side.
From EP 1 266 147 B1 a fluid engineering system that can be operated by compressed air is also known that has a safety function.
From DE 10 2010 041 203 A1 an arrangement and a method for feeding an excipient are known, wherein a compressed-air system is used, which in one embodiment has two valves, the outputs of which are connected by the interposing of a shuttle valve with one and the same working cylinder.
DE 10 2004 042 891 B3 discloses a safety circuit having a plurality of solenoid valves, the functional reliability of which can be checked by means of electrical encoders.
DE 10 2009 025 502 B4 describes a pneumatic valve device, providing a venting cross-section that is greater than the aerating cross-section.
Current safety standards require cyclical examination that two-channel venting of a compressed-air system is being guaranteed. This is against a background in which a wide variety of defect scenarios are conceivable and known, which can lead to a breakdown of both channels or both working valves. An example here would be the sticking of the valve member gasket with the resultant impossibility to switch over the working valve or a rupture of the return spring with the resultant blocking of the current switching position of the working valve. This problem is particularly pronounced in compressed-air systems which are used to operate loads which are switched on and have to be supplied with compressed air for long periods. Here, though, the additional problem arises that an interruption in the operation of the load to check the functional capability of the working valves is not really acceptable, because this is associated with an interruption in the process performed by the load.
Consideration has therefore previously been given to checking the working valves while in operation, by only operating them until the valve member briefly performs just a limited partial stroke. In this way only slight venting takes place, causing only minor disruption to the load. In sensitive loads, however, such a partial stroke test can also result in an error message and a possible operational interruption.
Furthermore, the partial stroke examination does not provide any indication that the working valve is actually in a position to fully switch over to the venting position if necessary.